Japanese Patent Application Laid-Open (JP-A) No. 2008-293922 (Patent Document 1) discloses a conventional keyboard structure. The keyboard structure disclosed in Document 1 has a water-proof function wherein a movable plate, which is movable together with a key top, is disposed below a key cap and a sheet-shaped elastic member is disposed between the key top and the movable plate. In the keyboard structure, a link mechanism, which includes two link members, a rubber dome, a membrane sheet, and a back plate, is disposed below the movable plate. A water-proof property is realized for the link mechanism by covering the link mechanism with the sheet-shaped elastic member.
As a reference to a keyboard structure having an illumination function, there is JP-A No. 2008-235065 (Patent Document 2) disclosing a structure in which a back plate, which supports a membrane sheet, is imparted with light permeability and light reflectivity to exhibit uniform illumination with a simple structure, so that a back plate functions as a light guiding and a substrate. When light is incident from an LED light source disposed to face the back plate, the back plate directs the incident light toward the key top. This causes a character or a symbol on the key top to be illuminated.